Inherent in the operation of gas turbine engines for helicopters and aircraft is the possibility of airborne particles which are drawn into the turbine engine. The common problem in operating helicopters is the stirring up of sand clouds by the main rotors when hovering in close proximity to ground surfaces. The extremely high forces and turbulence of air driven by the rotors stirs up enough dust, sand and small pebbles to cause serious damage to helicopter gas turbine engines. Other commonly known problems are caused by hail, ice sheets and birds.
Large foreign particles can be prevented from entering the engine with screens, however, small sand particles are generally dealt with using an external inertial particle separator immediately upstream of the engine intake. Airborne sand particles are removed through centrifugal force in the inertial particle separator. The particle separator generally comprises an external main intake duct which is curved in order to force airborne particles radially outwardly into a bypass duct for discharge from the engine. Of the total airflow drawn into the inertial particle separator, a substantial fraction such as 15-50% forced through the bypass duct together with the centrifugally driven airborne particles. The remaining 50%-85% of the airflow then proceeds to the compressor section with substantially all of the airborne particles removed.
U.S. Pat. Nos. 4,509,962 and 4,592,765 to Breitman et al show two examples of forward mounted axial intake separators. These prior art separators intake particle contaminated air in an axial annular flow path. The intake has a single or double (S-shaped) sharp bend. Airborne particles continue travelling in a straight line while the air flows around the bend or bends. As a result, the particles bounce and deflect against inner and outer walls of the intake into a bypass duct for discharge from the engine. The shape of the inner and outer walls may be parabolic to focus the spray of deflected particles in the desired direction towards the bypass duct.
A significant disadvantage of these prior art separators is the additional space consummed. In effect the forward intake of the engine is extended and the overall length of the engine may be increased. In the case of radial intake engines, the axial extent of the engine is increased with the addition of a prior art particle separator.
Also included in prior art inertial particle separators are mechanically actuated deflector flaps to force the air to make a sharper bend at increased velocity, thereby increasing the centrifugal separation of airborne particles, at the cost of an increased pressure loss to the air delivered to the compressor.
Inertial particle separators of the prior art are commonly included in relatively bulky ducts which are mounted laterally external to the engine in the case of a radial inlet or towards the front of the engine in the case of an axial inlet. Whether mounted externally or forwardly, the prior art external inertial particle separators represent a weight increase and size increase for the engine and nacelle. Particularly in the case of lightweight helicopters, the increase in size and weight can be critical. For example, the external inertial particle separator can result in approximately 5.degree. additional power loss from the engine. To maintain the same output power to the rotors, a larger engine must be provided. The larger engine itself includes a weight and size penalty, in addition to increased fuel consumption.
In the prior art of axial inlet inertial particle separators, heating to prevent blockage by ice forming in the inlet is necessary. This is normally provided by equipping the separator with double walls and circulating hot compressor discharge air between the double walls. The hot air flow is controlled by a shut-off valve. However, the net result is that anti-icing provisions represent a considerable cost, weight and reliability penalty for the engine.
In summary therefore, although external inertial particle separators are successful in removing damaging sand particles from the air intake into an engine, there are significant disadvantages involved in their use.
A further disadvantage of conventional external inertial particle separators is the effort required on the behalf of technical personnel to coordinate the design and maintenance of large external ducts between the engine manufacturer and others involved in helicopter and aircraft production.
It is an object of this invention to substantially reduce the size and weight of an inertial particle separator by incorporating the separator with a radial intake of a gas turbine engine.
It is a further object of this invention to provide a radial intake gas turbine engine which includes a built-in inertial particle separator thereby eliminating the requirement of external duct work and the disadvantages of the prior art.
It is a further object of the invention to provide anti-icing capability in addition to the inertial particle separation function.